The lifetime prevalence of bipolar disorder is roughly 4.4 percent for U.S. adults when the 2.4 percent of people are included whose symptoms do not quite reach diagnostic threshold, according to an article in the May issue of Archives of General Psychiatry.

Previously, researchers had estimated that about 1 percent of adults had bipolar disorder. However, evidence indicates that current diagnostic criteria may be too narrow to effectively detect bipolar disorder in the general population, and that a broader definition of bipolar spectrum disorder would identify many more individuals with bipolar symptoms, according to the authors.

Kathleen R. Merikangas, PhD, National Institute of Mental Health, Bethesda, Maryland, and colleagues estimated the national prevalence of bipolar disorder using data from 9,282 individuals chosen to represent the general population.

Interviews were conducted between February 2001 and April 2003 to assess the presence of bipolar disorder and other psychiatric conditions. Participants were classified as having bipolar disorder I, characterized by at least one episode of mania and one of depression; bipolar disorder II, requiring an episode of depression plus hypomania, a milder form of mania that does not require hospitalization; and a milder, sub-threshold bipolar disorder that involves hypomania with or without depression, otherwise classified as bipolar disorder "not otherwise specified" in the current diagnostic nomenclature of the American Psychiatric Association.

The study found that 1.0 percent of participants had bipolar disorder I and 1.1 percent had bipolar disorder II in their lifetimes; in the previous 12 months, 0.6 percent had bipolar disorder I, 0.8 percent bipolar disorder II, and 1.4 percent had sub-threshold manifestations of bipolar disorder.

Symptoms began at age 18.2 years for bipolar disorder I, 20.3 years for bipolar disorder II and 22.2 years for sub-threshold bipolar disorder.

Roughly 95.8 to 97.7 percent of those with bipolar disorder and 88.4 percent of those with sub-threshold bipolar disorder also had another psychiatric condition such as an anxiety disorder or substance use disorder, which is a frequent complication of bipolar illness.

The majority of those with lifetime bipolar disorder (80.1 percent) received treatment, including 69.3 percent of those with sub-threshold bipolar disorder; most of those people sought treatment for depression rather than for manic symptoms.

However, over the previous 12 months, only 25 percent of those with bipolar disorder I, 15.4 percent with bipolar disorder II, and 8.1 percent with sub-threshold bipolar disorder received appropriate medication.

"The present results reinforce the argument of other researchers that clinically significant sub-threshold bipolar disorder is as least as common as threshold bipolar disorder," the authors wrote. "Although most individuals with bipolar disorder receive treatment owing to co-morbid disorders, the lack of recognition of their underlying bipolarity leads to only a few receiving appropriate treatment."

The findings suggest that a substantial proportion of those diagnosed with major depression may actually have a form of bipolar disorder. More individuals with other psychiatric disorders should also be screened for bipolar disorder, the authors concluded.

"Additional research is needed to resolve uncertainty regarding the most appropriate threshold and boundary distinctions for bipolar disorder. This uncertainty remains a major impediment to advancing the understanding of the bipolar disorder spectrum in the population."